The invention is intended for use specifically in so-called trunked networks, which are typically company networks or authority networks, in which all the channels are used by one or more companies or authority organisations. The subscribers of these networks have, in addition to the numbers of their own, group numbers indicating to which communication group or subscriber group the subscriber in question belongs so that the calls addressed to subscribers of the group in question can be transmitted to subscribers of that group.
The invention is suited for use in mobile communications systems the radio path of which is either digital or analog. Digital mobile communications systems may include, for instance, systems in accordance with the TETRA system (TETRA=Trans European Trunked Radio System). Analog mobile communications systems have been disclosed, e.g., in the following publications of The Department of Trade and Industry in Britain: "MPT 1327, January 1988, Revised and reprinted November 1991, A Signalling Standard for Trunked Private Land Mobile Radio Systems, Radiocommunications Agency" and "MPT 1343, January 1988, Revised and reprinted September 1991, Performance Specification, Radiocommunications Agency".
An essential feature of the above-mentioned mobile communications systems, for instance, is the capability and the purpose to carry out a group call between several of subscribers.
A group call is a conference call in which all the participants can speak and listen to each other in turn. In a group call, the entire group is called with one call number. A single mobile station (a radio telephone, for instance), that is, a subscriber may belong to several groups that are programmed in the mobile station. The system stores a file of base stations associated with the number of each group. A group call may cover one, several or all the base stations located within the area of a radio telephone exchange or a mobile services switching center, as well as several mobile services switching centers. When a group call is being established, a traffic channel is reserved from all the base stations belonging to the group, and each of these base stations sends a group call request containing a group number and information on the reserved traffic channel. If the mobile station identifies the group number contained in the group call request, it shifts onto the traffic channel indicated by the group call request.
In prior art mobile communication systems, when mobile stations participating in an individual call or in the above-mentioned group call are moving from one cell to another particularly in a trunked mobile communication system, the mobile station must interrupt its call. This is a severe problem particularly in trunked PMR (Private Mobile radio) networks, which are often intended to be used by authorities. Authorities, especially police-officers and rescue organisations in emergency duties often need a group call that extends over the area of several cells of the mobile communication system. A possibility of an uninterrupted call must be provided for a mobile station in a situation in which the mobile station is moving from one cell to another.
Previously, e.g., in the GSM system (GSM=Global System for Mobiles) the problem has been solved by means of measuring methods. In such a case, a radio unit, for instance, measures the field strength of the transmission of a base station, and, if the field strength is too low and a mobile station finds a base station with a higher field strength, it is registered to said new base station. In that situation, however, the call may be interrupted if the exchange software of the mobile communication system has not received information on the change of the area in advance. The measurements to be carried out in this procedure demand calculation capacity and accurate cooperation of the network and the mobile stations connected to it.
In the prior art solution set out above, when a subscriber shifts from one base station to another, the problem is the fact that the speech connection may be interrupted. In order that the following location could be estimated, complicated measuring methods would be required that are able to detect how the field strengths received by the receivers of a base station and a mobile station weaken and strengthen, and when it is advantageous for a mobile station to change over to communicating with another base station, that is, when a so-called handover should be performed. In addition to measuring methods, intelligent software solutions would be necessary by means of which solutions it would be possible to predict the following location in a reliable manner in situations in which a mobile station is moving fast within the service area of the mobile communication system from the coverage area of the first base station to the coverage area of some other base station.
The principal users of PMR and trunked networks, that is, authorities, e.g., police-officers and rescue staff need mobile individual and group calls in chase situations, for instance (chase group call). In such a case, a demand made on a mobile communication system is the fact that the call should not be interrupted at any stage; Furthermore, if such a group call is established that is not interrupted when a mobile station shifts from the coverage area of one base station to the coverage area of another base station, the solution to the problem must be completely reliable as authority networks require a maximum reliability owing to chase and emergency situations.
If the above-mentioned problem were solved by using prior art methods, the reliability of the solution would be too low for authorities using the system. In addition, the exchange software would become complicated and heavy, which would contradict the basic principle of trunked and PMR networks, that is, simplicity and cost-effectiveness. Furthermore, the fact that the software would become more extensive and more complicated would also reduce the reliability of the system. It is obvious that the more complicated the program, the higher the probability of a malfunction is. Such malfunctions must be eliminated from emergency and rescue work as carefully as possible.